if it is in the name of vivek then you need to check whether that firm is proprietorship concern or a partnership???
you better create a new partnership firm so that the understanding between your brother and vivek are covered in it. since the first company/firm is run by vivek and his father, i doubt your brother will get any power in that firm as he would be working as a manger only and all the decision making power shall rest in vivek and his father's hand only.
in that firm, your brother would not even have the authority to sign the cheque and do the bank transaction.
Therefore, in the interest of your borhter it is advisable to form a new firm and do the business. by creating a partneship deed.
Advocate Rohit Dalmia